Brush and brushless DC motors have been used frequently in battery-supplied applications, such as automotive, electric vehicle, forklift, etc. Damage to the motor can be caused by continuing to energize the motor in the event of a locked rotor, or reduced speed conditions that can result from full or partial obstruction of the motor rotor. Under these conditions, damage to the motor or the control mechanism for the motor itself can be the result.
In the control of a DC motor, or a DC brushless motor using electronic control means, variable speed is typically achieved through the use of Pulse Width Modulation (PWM). The electronic controller typically receives a control signal (analog or digital) through a control lead to vary the duty cycle of the PWM voltage applied to the motor windings. As a result, the motor speed will vary in accordance with speed control signal. A typical motor speed vs. command signal input is shown in FIG. 1, where the control signal is an actual low frequency PWM signal where the desired motor speed is determined by the positive or negative duty cycle.
In addition, in a typical PWM driven motor, current, and or temperature of power stage components are also measured in order to determine if the power stage components should be shut down due to an overload condition to protect either the motor, or control electronics. The system voltage can also be measured in order to shut down the motor in the event of an overvoltage or undervoltage condition. For example, in one typical application, it is desired to operate the motor from +9 V to +16 V. Within this range of operational voltages, a separate motor speed vs. control signal duty cycle will be present at any discrete voltage value. This relationship is shown in FIG. 2.
In some DC motor systems, the motor speed is also measured to indicate whether or not the motor has experienced a locked rotor condition. In many motor control systems, current and/or temperature are used to determine whether of not the motor has experienced an overload condition. Due to the characteristics of the temperature/current measuring conditioning circuitry, however, damage may occur to the motor and/or control circuitry due to the potentially long time constants associated with conditioning circuits. As such, current protection schemes may not protect the motor or control electronics under all foreseeable overload conditions. In addition, the additional circuitry associated with current/temperature limiting circuitry may increase the overall system cost due to the additional components required, as well as the process steps required to ensure repeatable, reliable operation of these circuits across production lots, and all operational conditions.
Accordingly, there is a need to provide an overload protection scheme that monitors motor speed at a particular voltage in order to provide reliable protection for both motor and control electronics across the entire operational range of the motor.